Method of reducing coupling between floating gates in nonvolatile memory

ABSTRACT

A nonvolatile memory array includes floating gates that have an inverted-T shape in cross section along a plane that is perpendicular to the direction along which floating cells are connected together to form a string. Adjacent strings are isolated by shallow trench isolation structures.

PRIORITY DATA

This application is a divisional of U.S. patent application Ser. No.11/534,135 entitled “METHOD OF REDUCING COUPLING BETWEEN FLOATING GATESIN NONVOLATILE MEMORY,” filed on Sep. 21, 2006, now pending.

BACKGROUND OF THE INVENTION

This invention relates generally to non-volatile flash memory systems,and, more specifically, to the structures of memory cells and arrays ofmemory cells, and to the process of forming them. All patents, patentapplications and other documents cited in the present application arehereby incorporated by reference in their entirety.

There are many commercially successful non-volatile memory productsbeing used today, particularly in the form of small form factor cards,which use an array of flash EEPROM (Electrically Erasable andProgrammable Read Only Memory) cells. In one type of architecture, aNAND array, a series of strings of more than two memory cells, such as16 or 32, are connected along with one or more select transistorsbetween individual bit lines and a reference potential to form columnsof cells. Word lines extend across cells within a large number of thesecolumns. An individual cell within a column is read and verified duringprogramming by causing the remaining cells in the string to be overdriven so that the current flowing through a string is dependent uponthe level of charge stored in the addressed cell. An example of a NANDarchitecture array and its operation as part of a memory system is foundin U.S. Pat. No. 6,046,935.

In another type of array having a “split-channel” between source anddrain diffusions, the floating gate of the cell is positioned over oneportion of the channel and the word line (also referred to as a controlgate) is positioned over the other channel portion as well as over thefloating gate. This effectively forms a cell with two transistors inseries, one (the memory transistor) with a combination of the amount ofcharge on the floating gate and the voltage on the word line controllingthe amount of current that can flow through its portion of the channel,and the other (the select transistor) having the word line alone servingas its gate. The word line extends over a row of floating gates.Examples of such cells, their uses in memory systems and methods ofmanufacturing them are given in U.S. Pat. Nos. 5,070,032, 5,095,344,5,315,541, 5,343,063, 5,661,053, and 6,281,075.

A modification of this split-channel flash EEPROM cell adds a steeringgate that provides a strong capacitive coupling to the floating gatewithout having direct control of the channel. Each steering gate of anarray extends over one column of floating gates, perpendicular to theword line. The effect is to relieve the word line from having to performtwo functions at the same time when reading or programming a selectedcell. Those two functions are (1) to serve as a gate of a selecttransistor, thus requiring a proper voltage to turn the selecttransistor on and off, and (2) to drive the voltage of the floating gateto a desired level through an electric field (capacitive) couplingbetween the word line and the floating gate. It is often difficult toperform both of these functions in an optimum manner with a singlevoltage. With the addition of the steering gate, the word line need onlyperform function (1), while the added steering gate performs function(2). For source side injection programming, efficient programming isobtained by driving the select gate just barely (by for example 0.5V)about its threshold voltage, whereas the steering gate voltage will beincrementally increased from one programming pulse to the next, withverify and lockout operation performed in between programming pulses.The use of steering gates in a flash EEPROM array is described, forexample, in U.S. Pat. Nos. 5,313,421 and 6,222,762.

In any of the types of memory cell arrays described above, the floatinggate of a cell is programmed by injecting electrons from the substrateto the floating gate. This is accomplished by having the proper dopingin the channel region and applying the proper voltages to the source,drain and remaining gate(s).

Two techniques for removing charge from floating gates to erase memorycells are used in the three types of memory cell arrays described above.One is to erase to the substrate by applying appropriate voltages to thesource, drain and other gate(s) that cause electrons to tunnel through aportion of a dielectric layer between the floating gate and thesubstrate. The other erase technique is to transfer electrons from thefloating gate to another gate through a tunnel dielectric layerpositioned between them. In the second type of cell described above, athird erase gate is provided for that purpose. In the third type of celldescribed above, which already has three gates because of the use of asteering gate, the floating gate is erased to the word line, without thenecessity to add a fourth gate. Although this latter technique adds backa second function to be performed by the word line, these functions areperformed at different times, thus avoiding the necessity of making acompromise because of the two conflicting requirements. When eithererase technique is utilized, a large number of memory cells are groupedtogether for simultaneously erasure, in a “flash.” In one approach, thegroup includes enough memory cells to store the amount of user datastored in a disk sector, namely 512 bytes, plus some overhead data. Inanother approach, each group contains enough cells to hold severalthousand bytes of user data, equal to many disk sectors' worth of data.Multi-block erasure, defect management and other flash EEPROM systemfeatures are described in U.S. Pat. No. 5,297,148.

As in most integrated circuit applications, the pressure to shrink thesilicon substrate area required to implement some integrated circuitfunction also exists with flash EEPROM systems. It is continuallydesired to increase the amount of digital data that can be stored in agiven area of a silicon substrate, in order to increase the storagecapacity of a given size memory card and other types of packages, or toboth increase capacity and decrease size. One way to increase thestorage density of data is to store more than one bit of data per memorycell. This is accomplished by dividing a window of a floating gatecharge level voltage range into more than two states. The use of foursuch states allows each cell to store two bits of data, eight statesstores three bits of data per cell, and so on. A multiple state flashEEPROM structure and operation is described in U.S. Pat. Nos. 5,043,940and 5,172,338.

Increased data density can also be achieved by reducing the physicalsize of the memory cells and/or the overall array. Shrinking the size ofintegrated circuits is commonly performed for all types of circuits asprocessing techniques improve over time to permit implementing smallerfeature sizes. But there are usually limits of how far a given circuitlayout can be shrunk in this manner, since there is often at least onefeature that is limited as to how much it can be shrunk, thus limitingthe amount that the overall layout can be shrunk. When this happens,designers will turn to a new or different layout or architecture of thecircuit being implemented in order to reduce the amount of silicon arearequired to perform its functions. The shrinking of the above-describedflash EEPROM integrated circuit systems can reach similar limits.

Another flash EEPROM architecture utilizes a dual floating gate memorycell along with the storage of multiple states on each floating gate. Inthis type of cell, two floating gates are included over its channelbetween source and drain diffusions with a select transistor in betweenthem. A steering gate is included along each column of floating gatesand a word line is provided thereover along each row of floating gates.When accessing a given floating gate for reading or programming, thesteering gate over the other floating gate of the cell containing thefloating gate of interest is raised sufficiently high to turn on thechannel under the other floating gate no matter what charge level existson it. This effectively eliminates the other floating gate as a factorin reading or programming the floating gate of interest in the samememory cell. For example, the amount of current flowing through thecell, which can be used to read its state, is then a function of theamount of charge on the floating gate of interest but not of the otherfloating gate in the same cell. Examples of this cell array architectureand operating techniques are described in U.S. Pat. Nos. 5,712,180,6,103,573 and 6,151,248.

In these and other types of non-volatile memories, the amount of fieldcoupling between the floating gates and the control gates passing overthem is carefully controlled. The amount of coupling determines thepercentage of a voltage placed on the control gate that is coupled toits floating gates. The percentage coupling is determined by a number offactors including the amount of surface area of the floating gate thatoverlaps a surface of the control gate. It is often desired to maximizethe percentage coupling between the floating and control gates bymaximizing the amount of overlapping area. One approach to increasingcoupling area is described by Yuan et al in U.S. Pat. No. 5,343,063. Theapproach described in that patent is to make the floating gates thickerthan usual to provide large vertical surfaces that may be coupled withthe control gates. Another approach that increases area coupling afloating gate and a control gate is described by Yuan in U.S. Pat. No.6,908,817.

When increasing the vertical coupling areas between adjacent floatingand control gates, it is further desirable to do so in a manner thatdoes not increase the area of the substrate that is occupied by eachcell. Also, it is preferable to reduce the floating gate to floatinggate coupling, so that adjacent floating gates do not greatly affecteach other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a nonvolatile memory system including a controller and anarray of memory cells in which various embodiments of the presentinvention may be utilized.

FIG. 2A shows a top-down view of a NAND flash memory array according toan embodiment of the present invention.

FIG. 2B shows an individual floating gate of the NAND flash memory arrayof FIG. 2A having an inverted-T shape in cross section.

FIG. 3 shows a cross section of the NAND flash memory array of FIG. 2Aat an early stage of fabrication with slimmed photoresist portionsoverlying a masking layer that overlies a substrate.

FIG. 4 shows the structure of FIG. 3 after patterning the masking layerinto masking portions that are then used to locate STI trenches that arefilled with Silicon Dioxide.

FIG. 5 shows the structure of FIG. 4 after planarization to removemasking portions and excess Silicon Dioxide.

FIG. 6 shows the structure of FIG. 5 after formation of a gatedielectric layer, a first floating gate layer, masking portions andsidewall spacers on exposed sidewalls of masking portions.

FIG. 7 shows the structure of FIG. 6 after dividing the first floatinggate layer into first floating gate portions with dielectric betweenthem and removal of masking portions and sidewall spacers.

FIG. 8 shows the structure of FIG. 7 after formation of masking portionsand sidewall spacers to form slots over floating gate portions.

FIG. 9 shows the structure of FIG. 8 after deposition of a secondfloating gate layer that fills slots over first floating gate portions.

FIG. 10 shows the structure of FIG. 9 after removal of excess secondfloating gate material, masking portions and sidewall spacers.

FIG. 11 shows the structure of FIG. 10 after formation of a dielectriclayer and control gate layer over floating gate portions.

FIG. 12 shows a flowchart of the process of FIGS. 3-11.

FIG. 13 shows a cut-away view of the structure of FIG. 11 includingseparate word lines extending over floating gates with control gates andfloating gates self-aligned.

FIG. 14 shows certain dimensions of the structures of FIG. 13.

FIG. 15 shows misalignment between floating gates and channel regions ina memory array.

FIG. 16 shows misalignment between a lower floating gate portion and anupper floating gate portion of a floating gate.

FIG. 17 shows a cross section of a NAND flash memory array at an earlystage of fabrication, according to another embodiment of the presentinvention, with STI structures and first floating gate portions that areformed in a self-aligned manner.

FIG. 18 shows the structure of FIG. 17 after formation of sidewallspacers on exposed sidewalls of STI structures so that slots are formedover first floating gate portions, cavities are also formed at the baseof slots.

FIG. 19 shows the structure of FIG. 18 after deposition of a secondfloating gate layer that fills slots between sidewall spacers and fillscavities.

FIG. 20 shows the structure of FIG. 19 after planarization to removeexcess second floating gate material.

FIG. 21 shows the structure of FIG. 20 after removal of sidewall spacersand removal of portions of STI structures.

FIG. 22 shows the structure of FIG. 21 after deposition of a dielectriclayer and deposition of a control gate layer over floating gates.

FIG. 23 shows a flowchart of the process described in FIGS. 17-22.

FIG. 24 shows a cross section of a NAND flash memory array at an earlystage of fabrication, according to another embodiment of the presentinvention, with masking portions establishing a pattern for partiallyetching a floating gate layer.

FIG. 25 shows the structure of FIG. 24 after oxidation of the exposedfloating gate layer and masking portions.

FIG. 26 shows the structure of FIG. 25 after formation of sidewallspacers from the oxide layer and formation of STI trenches using thesidewall spacers to establish the locations of trenches.

FIG. 27 shows the structure of FIG. 26 after sidewall spacers andmasking portions are removed and STI trenches are filled.

FIG. 28 shows the structure of FIG. 27 after deposition of a dielectriclayer and control gate layer.

FIG. 29 shows a flowchart of the process described in FIGS. 24-27.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Memory Operation

An example of a memory system 100 incorporating the various aspects ofthe present invention is generally illustrated in the block diagram ofFIG. 1. A large number of individually addressable memory cells arearranged in a regular array 110 of rows and columns, although otherphysical arrangements of cells are certainly possible. Bit lines,designated herein to extend along columns of the array 110 of cells, areelectrically connected with a bit line decoder and driver circuit 130through lines 150. Word lines, which are designated in this descriptionto extend along rows of the array 110 of cells, are electricallyconnected through lines 170 to a word line decoder and driver circuit190. Each of the decoders 130 and 190 receives memory cell addressesover a bus 160 from a memory controller 180. The decoder and drivingcircuits are also connected to the controller 180 over respectivecontrol and status signal lines 135 and 195.

The controller 180 is connectable through lines 140 to a host device(not shown). The host may be a personal computer, notebook computer,digital camera, audio player, various other hand held electronicdevices, and the like. The memory system 100 of FIG. 1 will commonly beimplemented in a removable card according to one of several existingphysical and electrical standards, such as one from the PCMCIA, theCompactFlash™ Association, the MMC™ Association, and others. Otherremovable formats include USB flash drives such as Cruzer® flash drives.When in a removable format, the lines 140 terminate in a connector thatinterfaces with a complementary connector of the host device. Theelectrical interface of many removable memory systems follows the ATAstandard, wherein the memory system appears to the host as if it was amagnetic disk drive. Other memory card interface standards also exist.As an alternative to the card format, a memory system of the type shownin FIG. 1 may be permanently embedded in the host device.

The decoder and driver circuits 130 and 190 generate appropriatevoltages in their respective lines of the array 110, as addressed overthe bus 160, according to control signals in respective control andstatus lines 135 and 195, to execute programming, reading and erasingfunctions. Any status signals, including voltage levels and other arrayparameters, are provided by the array 110 to the controller 180 over thesame control and status lines 135 and 195. A plurality of senseamplifiers within the circuit 130 receive current or voltage levels thatare indicative of the states of addressed memory cells within the array110, and provides the controller 180 with information about those statesover lines 145 during a read operation. A large number of senseamplifiers are usually used in order to be able to read the states of alarge number of memory cells in parallel. During reading and programoperations, one row of cells is typically addressed at a time throughthe circuits 190 for accessing a number of cells in the addressed rowthat are selected by the circuit 130. During an erase operation, allcells in each of many rows are typically addressed together as a blockfor simultaneous erasure.

A plan view of an example of a NAND memory cell array 110 formed on asilicon substrate is shown in FIG. 2(A), wherein a small part of itsrepetitive structure of conductive elements is illustrated with littledetail of dielectric layers that exist between the elements, for clarityof explanation. Shallow Trench Isolation (STI) structures 210 a-d areformed extending through the surface of the substrate. In order toprovide a convention for this description, the STI structures are shownto be spaced apart in a first x-direction, with lengths extending in asecond y-direction, these first and second directions being essentiallyorthogonal with each other.

Between STI structures 210 a-d, there are strings 220 a-c of memorycells running in the y-direction. Thus, the direction of the strings isparallel to the direction of the STI structures. Each of strings 220 a-cincludes many memory devices connected in series. FIG. 2(A) showsportions of three strings 220 a-c with three memory cells shown for eachstring. However, strings 220 a-c may contain additional cells that arenot shown in FIG. 2(A). Also, the array 110 contains additional stringsthat are not represented in FIG. 2(A). This type of array may havethousands of strings with 16, 32 or more cells in each string.

An exemplary memory cell includes a floating gate 230 and conductivesource/drain regions 240 a-b in the substrate adjacent to floating gate230, on either side in the y-direction. Strings are separated by STIstructures 210 a-d. STI structures 210 a-d form isolating elements thatelectrically isolate source/drain regions from source/drain regions ofcells in adjacent strings. Along the y-direction source/drain regionsare shared by adjacent cells. The source/drain regions electricallyconnect one cell to the next cell thus forming a string of cells. Thesource/drain regions 240 a-c in this example are formed by implantingimpurities into the substrate in the required areas.

Floating gates shown in the embodiment of FIG. 2(A) comprise twoportions that can be better seen in the three dimensional view offloating gate 230 a shown in FIG. 2(B). A lower floating gate portion231 extends across the surface of the substrate on a thin silicondioxide (oxide) layer. An upper floating gate portion 232 projectsupward from upper surface 233 of lower floating gate portion 231 to forman inverted-T shape in cross section along the X-direction. Upperfloating gate portion 232 extends to the edges of lower floating gateportion 231 in the y-direction but is narrower in the x-direction. Thus,the floating gate is wider below an intermediate level than it is abovethe intermediate level, leaving some of upper surface 233 of lowerfloating gate portion 231 exposed.

Lower and upper floating gate portions 231, 232 of this embodiment areboth made of doped polysilicon. Polysilicon may be deposited in anundoped form and later implanted to form doped polysilicon or may bedeposited in doped form. In one embodiment, lower floating gate portion231 is deposited as undoped polysilicon and upper floating gate portion232 is deposited as doped polysilicon. Later, after being subject toelevated temperatures for some period during processing, dopant fromupper floating gate portion 232 diffuses into lower floating gateportion 231 so that it too becomes doped and conductive. Other suitableelectrically conductive materials may also be used in place of dopedpolysilicon. Lower floating gate portion 231 and upper floating gateportion 232 may also be deposited in a single layer instead of twoseparate layers.

Word lines 250 a-c are shown extending across the array in thex-direction in FIG. 2(A). Word lines 250 a-c overlie portions offloating gate 230 a and also partially surround floating gate 230 a. Inthe embodiment shown, word line 250 b overlies the exposed parts ofupper surface 233 of lower floating gate portion 231 and encloses theupper surface and the sides of upper floating gate portion 232. Upperfloating gate portion 232 adds to the surface area of the floating gatethat couples floating gate 230 a and the control gate 250 b. Thisincreased area provides an improved coupling ratio compared to aconventional floating gate.

Not shown in FIG. 2(A) are metal conductor layers. Since polysiliconelements such as word lines usually have a conductivity that issignificantly less than that of metal, metal conductors are included inseparate layers with connections made to respective metal lines throughany intermediate layers at periodical intervals along the lengths of thepolysilicon elements. Also, the word line may include a metal ormetal-silicide portion to increase the electrical conductivity of theword line. For example, a refractory metal such as Cobalt or Tungstenmay be used to form a silicide layer on top of the polysilicon layer.The silicide material has a higher conductivity than the polysilicon andthus improves electrical conduction along the word line.

Process for Forming Inverted-T Floating Gate

FIG. 3 shows a cross sectional view of the NAND memory array 110 of FIG.2(A), along the X-direction (word line direction) indicated by I-I inFIG. 2(A), at an early stage of fabrication. A layer 301 of SiliconNitride (SiN) or other masking material extends over an upper surface303 of a substrate 305. Masking layer 301 may be deposited to cover theentire upper surface 303 and is therefore considered a blanket layer.Photoresist portions 307 a-c are formed over masking layer 301.Photoresist is generally spun on at high speed to form a blanket layerthat is then patterned by exposing the photoresist to light (or in somecases an electron beam), according to a predetermined pattern, in alithographic process. This pattern determines which portions are removedand which remain when the photoresist is developed. Photoresist portions307 a-c extend in the y-direction (perpendicular to the cross section ofFIG. 3) and overlie masking layer 301. Photoresist portions 307 a-c maybe formed to be very small and very closely spaced given the limits ofthe lithographic process used. In one example, the width of aphotoresist portion is equal to the minimum feature size F when it isdeposited, and the distance between adjacent photoresist portions is Fwhen they are deposited. However, in the present embodiment, photoresistportions 307 a-c are subject to a slimming process that removes somephotoresist, leaving photoresist portions 307 a-c having a width that isless than the minimum feature size. Examples of resist slimmingprocesses are described in U.S. Pat. No. 6,888,755 and U.S. patentapplication Ser. No. 11/316,654. Subsequently, photoresist portions 307a-c are used to pattern masking layer 301 into masking portions in thesame pattern as photoresist portions 307 a-c. Then, masking portions areused as an etch mask for forming trenches for isolation.

FIG. 4 shows masking portions 409 a-c patterned using photoresistportions 307 a-c and the trenches that are formed using masking portions409 a-c as an etch mask. Once masking layer 301 is patterned to formseparate masking portions 409 a-c, photoresist portions 307 a-c may beremoved. Then, an anisotropic etch is performed with masking portions409 a-c in place. The result is the formation of trenches into substrate305 between areas of substrate 305 that are covered by masking portions409 a-c. These trenches are then filled with Silicon Dioxide (SiO₂ or“oxide”) 411. Subsequently, masking portions 409 a-c and excess SiliconDioxide 411 may be removed (e.g. by CMP or using an etch-back process)to leave a planarized surface as shown in FIG. 5. The Silicon Dioxidefilled trenches form Shallow Trench Isolation (STI) structures 210 a-d.STI structures 210 a-d provide isolation between adjacent memory cells.STI structures 210 a-d extend in the Y-direction. Between adjacent STIstructures 210 a-d are remaining substrate portions 515 a-c where memorycells are later formed. Unlike some prior structures, STI structures 210a-d of FIG. 5 are wider than remaining substrate portions 515 a-cbetween them, and remaining substrate portions 515 a-c are narrower thanthe minimum features size of the lithographic process used.

Subsequent to the planarizing step, a gate dielectric layer 617 (in thisexample, Silicon Dioxide) is formed across substrate 305 (e.g. bythermal oxidation or deposition) and a first conductive layer 519 ofconductive material (in this example, polysilicon) is deposited overgate dielectric layer 617 as shown in FIG. 6. A masking layer isdeposited over first conductive layer 519 and is patterned into maskingportions 521 a-c that are located over remaining substrate portions 515a-c. Masking portions 521 a-c are Silicon Nitride in this example andare patterned using photoresist, though in this case without resistslimming. In order to locate masking portions 521 a-c over remainingsubstrate portions 515 a-c, the pattern forming masking portions 521 a-cis aligned to the existing pattern of STI structures 210 a-c andremaining portions 515 a-c. After masking portions 521 a-c are formed,sidewall spacers 523 a-f are formed along their sides. Such sidewallspacer formation is well known and may be achieved by depositing a layerof dielectric and then performing an anisotropic etch. Prior to formingsidewall spacers 523 a-f, the gap between masking portions 521 a-c isthe minimum feature size (F). Sidewall spacers 523 a-f reduce this gapso it is less than the minimum feature size. Masking portions 521 a-cwith sidewall spacers 523 a-f are then used as an etch mask to etchthrough the first conductive layer 519. The portions of first conductivelayer 519 that are removed by this etch are then replaced by a suitabledielectric (in this example, Silicon Dioxide).

FIG. 7 shows the structure of FIG. 6 after first conductive layer 519 isseparated into first conductive portions 519 a-c that extend in they-direction (perpendicular to the cross section of FIG. 7) and maskingportions 521 a-c and sidewall spacers 523 a-f are removed providing aplanarized surface. Removal of masking portions 521 a-c and sidewallspacers 523 a-f may also remove any excess dielectric so that onlydielectric portions 725 a-b lying between first floating gate portions519 a-c remain. First conductive portions 519 a-c are electricallyisolated from each other by dielectric portions 725 a-b and are isolatedfrom underlying remaining substrate portions 515 a-c by gate dielectriclayer 617. Subsequently, another masking layer is formed and patterned.

FIG. 8 shows the structure of FIG. 7 with second masking portions 827a-d (of Silicon Nitride) and sidewall spacers 829 a-f (of SiliconDioxide) formed over first conductive portions 519 a-c and dielectricportions 725 a-c. Masking portions 827 a-d and sidewall spacers 829 a-fmay be formed as before so that the gaps between sidewall spacers 829a-f are less than the minimum feature size and these gaps form slots 831a-c that extend upwards from first conductive portions 519 a-c. Formingmasking portions 827 a-d involves aligning the pattern used to formmasking portions 827 a-d to preexisting structures. In this case,masking portions 827 a-d are located over STI structures 210 a-d so thatslots 831 a-c between sidewall spacers are centered over firstconductive portions 519 a-c. Subsequently, a second floating gate layeris deposited to fill slots 831 a-c.

FIG. 9 shows the result of depositing a second conductive layer 933 tofill slots 831 a-c. The material of second conductive layer 933 is dopedpolysilicon in this example. Second conductive layer 933 is in contactwith first conductive portions 519 a-c at the bottom of slots 831 a-cand forms electrical contact at these points. After second conductivelayer 933 is deposited, an etch (or series of different etches) may beperformed to remove excess material of second conductive layer 933 andto remove masking portions 827 a-d and sidewall spacers 829 a-f.

FIG. 10 shows the result of the removal of excess material of secondconductive layer 933, masking portions 827 a-d and sidewall spacers 829a-f. Second conductive portions 933 a-c remain that extend upwards fromfirst conductive portions 519 a-c to form an inverted-T shape in thecross section shown in FIG. 10. Second conductive portions 933 a-cremain where slots 813 a-c were formed and their dimensions aredetermined by slots 813 a-c and may be less than the minimum featuresize. Subsequently, a second dielectric layer is deposited over thefirst and second conductive portions and a control gate layer isdeposited over the second dielectric layer.

FIG. 11 shows the structure of FIG. 10 with a second dielectric layer1135 overlying first conductive portions 519 a-c and second conductiveportions 933 a-c and with a control gate layer 1137 overlying seconddielectric layer 1135. The material of control gate layer 1137 is dopedpolysilicon in the present example. An additional layer of TungstenSilicide, Cobalt Silicide or other conductive material may also be addedover the polysilicon to provide a control gate layer with lower sheetresistance. Second dielectric layer 1135 may be referred to as InterPoly Dielectric (IPD), though in some cases, material other thanpolysilicon may be used for conductive portions or control gatematerial, or for both conductive portion material and control gatematerial. In the example of FIG. 11 second dielectric layer 1135 is acompound layer made up of a layer of Silicon Dioxide (oxide), then alayer of Silicon Nitride (nitride), then another layer of SiliconDioxide (oxide). This oxide-nitride-oxide (ONO) stack may provide betterperformance than a single dielectric material. Subsequent to formingcontrol gate layer 1137, a patterning step may be performed to separatecontrol gate layer 1137 into word lines 250 a-c and, in the same step,separate first and second conductive portions into separate floatinggates. In this way, floating gates and word lines are self-aligned.

FIG. 12 shows a flowchart that summarizes the process of FIGS. 3-11.First, a masking layer is formed on a substrate and a photoresist layeris formed over it 1241. The photoresist layer is patterned 1243 and thepattern is then subjected to resist slimming 1245. The slimmedphotoresist portions are then used to pattern the masking layer intomasking portions 1247. Masking portions form a mask layer that is usedto establish the locations of trenches. The trenches are filled withSilicon Dioxide and planarization is performed to form STI structures1249. A gate dielectric layer and a first floating gate layer are thendeposited 1251. Masking portions are formed over the first floating gatelayer 1253 and sidewall spacers are formed on the sides of the maskingportions 1255. Then, the masking portions and sidewall spacers are usedas a mask to etch the first floating gate layer 1257 and thus formseparated first floating gate portions. Dielectric is deposited to fillthe gaps between first floating gate portions and then planarization isperformed 1259 to remove excess dielectric, masking portions andsidewall spacers. Another set of masking portions and sidewall spacersare formed 1261 over the first floating gate portions. The maskingportions and sidewall spacers are located so that slots formed betweensidewall spacers overlie first floating gate portions. The slots arefilled with a second floating gate layer 1263 and then excess secondfloating gate material is removed along with masking portions andsidewall spacers 1265 leaving second floating gate portions. This leavessurfaces of first and second floating gate portions exposed.Subsequently a dielectric layer is deposited over the first and secondfloating gate portions and a control gate layer is deposited over thedielectric layer 1267. An etch step etches the stack formed by priorprocessing so that the control gate layer is divided into separate wordlines and the first and second floating gate portions are divided intoseparate floating gates 1269. Thus, floating gates are self-aligned toword lines.

FIG. 13 shows the structure of FIG. 2A in three dimensions. Word lines250 a-d extend in the x-direction and are spaced apart in theY-direction. Word lines 250 a-d form control gates where they overlieand are coupled to individual floating gates. STI structures 210 a-cextend in the Y-direction and are spaced apart in the X-direction. Wordlines 250 a-c may be used as an implant mask to implant source/drainregions between memory cells in the Y-direction. Such implantationconnects memory cells into strings that extend in the Y-directionbetween STI structures 210 a-c. Individual floating gates consist of alower portion and an upper portion as shown in FIG. 2B.

One advantage of the structure of FIG. 13 is that there is lesscapacitive coupling between adjacent floating gates along theY-direction. Some prior structures use a floating gate that isrectangular in cross section along the X-direction, with the lateraldimension determined by the minimum feature size of the lithographicprocess used. In contrast, the embodiment of FIG. 13 has floating gateswith an upper portion that is narrower than the minimum feature size.This means that area of the exposed facets of the floating gates of FIG.13 may be reduced, thereby reducing the capacitive coupling betweenadjacent floating gates. Reducing the facet area in this way may be donewithout reducing coupling between the control gate and floating gate.The coupling between control gate and floating gate depends on the totalarea of the control gate that overlies a surface of the floating gate.This is unaffected by making an upper portion of the floating gatenarrower in the X-direction. In addition, in the embodiment of FIG. 13,fringing fields between a floating gate and the control gate coupled toit may be increased relative to electric fields between the floatinggate and its neighbor in the Y-direction. This is because the distancebetween the control gate and the center of a facet of the floating gateis reduced compared with the distance between the facet and a facet ofan adjacent floating gate in the Y-direction. This tends to improvecoupling between the floating gate and the control gate compared withthe coupling between the floating gate and its neighbor in theY-direction.

An advantage of the structure of FIG. 13 is that it may be scaled tosmall dimensions more easily than some other structures. In particular,because the upper portions of floating gates are relatively narrow, thisleaves more space between floating gates at this level, so that there ismore room for the control gate and dielectric layer. Given certainlimitations in how small the control gate and dielectric layer can bemade, this may allow memory cells to be made smaller than they would beif the upper portion was larger. FIG. 14 illustrates this concept. Across section along a word line 250 c (i.e. along the X-direction) isshown with certain dimensions indicated. The distance between a point ona floating gate 230 x and a corresponding point on a neighboringfloating gate 230 y along the X-direction is twice the minimum featuresize (2 F). In FIG. 14, the distance 2 F is shown extending from theside of an upper portion of floating gate 230 x to the side of an upperportion of adjacent floating gate 230 y in the X-direction. Within thedistance 2 F there is an upper floating gate portion 1471 having athickness t₁ and a portion 1473 of word line 250 c having a thickness t₂and between them dielectric layer 1135 having a thickness t₃. Thus, inthis example, 2 F=t₁+t₂+2t₃. The dimensions, t₁, t₂ and t₃ may havecertain minimum values for any given materials in order to avoid highfailure rates. Typically, where an upper floating gate portion is formedof polysilicon, t₁ will not be made less than 100 Angstroms. Similarly,if a word line is formed of polysilicon, t₂ will not be made less than100 Angstroms. Where the dielectric is an ONO layer, t3 will typicallynot be made less than 120 Angstroms. So using these minimum values, 2F=440 Angstroms and F=220 Angstroms (22 nanometers). Thus, for certainmaterials, adequate performance may be maintained in the structure shownwith minimum feature sizes as small as 22 nanometers. In contrast, ift₁=F (upper portion has dimension equal to minimum feature size), thent₂+2t₃=F, and substituting the minimum values, F=340 Angstroms (34nanometers). While these examples relate to particular materials andtheir limits, where other materials are used, other limits may apply.

Another advantage of the embodiment of FIG. 13 is that it is relativelyinsensitive to misalignment that may occur between components. Forexample, if first floating gates 230 x-y are misaligned with respect toSTI structures 210 a-d this may not greatly affect device performance.FIG. 15 shows misalignment of δ₁ between first conductive portions 519a-c and channel regions that are formed from remaining substrateportions 515 a-c. Because first conductive portions 519 a-c are madewider than the minimum feature size and remaining portions 515 a-c aremade smaller than the minimum feature size, first conductive portions519 a-c still overlie the entire width of the channel regions and thereis no great change in coupling between first conductive portions andchannel regions in this case. In other embodiments, the channel regionmay be made smaller without making the first floating gate portionslarger and this may provide enough margin for error in alignment.Similarly, providing wide first floating gate portions without makingthe channel smaller may be sufficient.

FIG. 16 shows misalignment of δ₂ between a lower floating gate portion1675 and an upper floating gate portion 1677. In this case, the couplingbetween floating gate 1679 (formed of lower portion 1675 and upperportion 1677) and an overlying control gate remains the same because thearea that couples floating gate 1679 to the control gate is not affectedby moving the upper floating gate portion 1677 in the X-direction. Thus,the structure of FIG. 13 is relatively tolerant of misalignment.

Self-Aligned Process

An alternative process to that described above uses self-alignment toproduce features that do not need a separate alignment step to establishtheir relative locations. By not requiring separate alignment steps, theoverall process flow may be simplified and thus, costs may be reduced.In addition, failure due to misalignment may be reduced or eliminated.

FIG. 17 shows a cross section of a NAND memory array, along theX-direction, at an early stage of fabrication. STI structures 1701 a-dextend in the Y-direction and are shown in cross section in FIG. 17.Between STI structures 1701 a-d, gate dielectric portions 1703 a-d andfirst conductive portions 1705 a-c extend in the Y-direction. Thestructure shown in FIG. 17 is generally formed by depositing a blanketlayer of gate dielectric (in this case Silicon Dioxide) followed by ablanket layer of conductive material. In the present example, theconductive material is polysilicon deposited to a thickness of 10nanometers. Next, masking portions that extend in the Y-direction areformed over the floating gate layer. Trenches are formed according tothe pattern of the masking portions. The trenches extend through theconductive layer, gate dielectric layer and into the underlyingsubstrate. These trenches divide the conductive layer and the gatedielectric layer into first conductive portions 1705 a-c and gatedielectric portions 1703 a-c respectively. The trenches are filled withdielectric material, in this case Silicon Dioxide to form STI structures1701 a-d. Subsequently, masking portions are removed from over the firstconductive portions 1705 a-c to leave sidewalls of STI structures 1701a-d exposed. Subsequently, sidewall spacers are formed on STI sidewalls.

FIG. 18 shows the structure of FIG. 17 after formation of sidewallspacers 1807 a-f along exposed sidewalls of STI structures 1701 a-d.Sidewall spacers 1807 a-f are formed by depositing a layer of SiliconDioxide using a TEOS (Tetraethyl Orthosilicate) based process and thenperforming anisotropic etching. Sidewall spacers 1807 a-f overlie firstconductive portions 1705 a-c. Slots 1809 a-c are formed between sidewallspacers 1807 a-f over first conductive portions 1705 a-c so that firstconductive portions 1705 a-c are partially exposed. FIG. 18 shows someof first conductive portions 1705 a-c removed under slots 1809 a-c toform cavities 1811 a-c at these locations. Cavities 1811 a-c extendbetween sidewall spacers 1807 a-f and first conductive portions 1705a-c. Cavities 1811 a-c may be formed by performing an additional wetetch after the dielectric layer is etched to form slots 1809 a-c. Insome cases, no cavities are formed in first conductive portions, so nowet etch is needed. Subsequently, a second floating gate layer isdeposited.

FIG. 19 shows second conductive layer 1913 (of doped polysilicon)deposited over the structure of FIG. 18. In particular, secondconductive layer 1913 is deposited to fill cavities 1811 a-c in firstconductive portions 1705 a-c and to fill slots 1809 a-c that overliefirst conductive portions 1705 a-c. Second conductive layer 1913 alsooverlies STI structures 1701 a-d and sidewall spacers 1807 a-f Formingcavities 1811 a-c allows good adhesion between first conductive portions1705 a-c and second conductive layer 1913. In particular, when secondconductive layer 1913 is deposited, it fills cavities 1811 a-c and thisprovides a stable base for structures that are later formed. Theincreased interface area between the first conductive portions 1705 a-cand second conductive layer 1913 improves the physical strength of thebond between these portions. This may be important to avoid damageduring later processing. In particular, CMP or other processes couldcause second floating gate portions to break-off if they were notadequately secured. In some other examples, such cavities may not beused because sufficient contact is achieved without them.

FIG. 20 shows the structure of FIG. 19 after planarization to removeexcess material of second conductive layer 1913. This leaves secondconductive portions 1913 a-c attached to first conductive portions 1705a-c. This may be achieved by Chemical Mechanical Polishing (CMP) oretch-back or other means. This planarization may remove some materialfrom STI structures 1701 a-d and sidewall spacers 1807 a-f also.Subsequently, additional material is removed from STI structures 1701a-d and sidewall spacers 1807 a-f.

FIG. 21 shows the structure of FIG. 20 after removal of sidewall spacers1807 a-f and removal of portions of STI structures 1701 a-d down to alevel that is close to gate dielectric portions 1703 a-c. In some cases,portions of STI structures are removed down to a level that is lowerthan shown. For example, STI structures may be etched below the level ofthe top of the gate dielectric portions 1703 a-c. In other cases, STImaterial is removed down to a higher level than shown, such as the levelof the top of first conductive portions 1705 a-c. Subsequently, adielectric layer and control gate layer are formed over conductiveportions.

FIG. 22 shows the structure of FIG. 21 after deposition of a dielectriclayer 2215 and a control gate later 2217. These layers may be depositedas previously described and subsequently etched according to a patternto form word lines and separate floating gates in a self-aligned manner.Thus, a memory array is formed that is similar to that shown in FIG. 2A,with floating gates having an inverted-T shape as shown in FIG. 2B. Inthis example, a portion 2219 of control gate layer 2217 extends downbetween adjacent first conductive portions 1705 a, 1705 b and thusprovides shielding between adjacent lower floating gate portions in theword line direction.

FIG. 23 shows a flowchart that summarizes the process of FIGS. 17-22.The first floating gate layer and gate dielectric layer are first formedas blanket layers 2321. Then, STI structures are formed 2323, thusseparating the first floating gate layer into separate portions thatlater form individual floating gates. Sidewall spacers are formed 2325on the exposed sides of STI structures overlying first floating gateportions so that slots remain over the first floating gate portions. Awet etch removes some of the exposed first floating gate material andsome floating gate material under sidewall spacers 2327. A secondfloating gate layer is deposited to fill the slots and cavities 2329.Then, excess second floating gate material is removed along with thesidewall spacers and portions of STI structures 2331. Then, a dielectriclayer and control gate layer are deposited over the floating gateportions STI structures 2333. A patterned etch is then performed to formseparate word lines and floating gates that are self-aligned 2335.

Sidewall Oxidation Process

In an alternative embodiment, inverted-T shaped floating gates areformed by shaping a conductive layer by removal of conductive materialand subsequently separating the conductive layer into separateconductive portions and forming STI trenches so that they areself-aligned to conductive portions.

FIG. 24 shows a cross section of a NAND memory array, along theX-direction, at an early stage of fabrication. A gate dielectric layer2402 (of Silicon Dioxide in this example) is present on substrate 2400and a conductive layer 2404 (of doped polysilicon in this example)overlies gate dielectric layer 2402. Conductive layer 2404 may bedeposited in a single step to form a uniform layer, or may be depositedin more than one step so that conductive layer 2404 includes, forexample, polysilicon of different doping levels in different layers.Masking portions 2406 a-c (of Silicon Nitride in this example) extendover conductive layer 2404 in the Y-direction (perpendicular to thecross section shown). Conductive layer 2404 is shaped by etching in thepattern of masking portions 2406 a-c. In this case, conductive layer2404 is not etched all the way through to underlying gate dielectriclayer 2402. Etching away of portions of conductive layer 2404 may bedone by Reactive Ion Etching (RIE) or some other anisotropic etchingmeans. The width of masking portions 2406 a-c of FIG. 24 may be equal tothe minimum feature size of the lithographic process used or may be lessin some cases. Resist slimming or other means may be used to reduce thewidth of masking portions 2406 a-c. Accordingly, the spaces betweenadjacent masking portions 2406 a-c may be the minimum feature size (F)or may be larger. When etching has removed portions of conductive layer2404, vertical projections 2408 a-c are formed by remaining portions ofconductive layer 2404 that are covered by masking portions 2406 a-c.Vertical projections 2408 a-c later form upper portions of floatinggates.

FIG. 25 shows the structure of FIG. 24 after oxidation is performed togrow a Silicon Dioxide layer 2510 on exposed surfaces of conductivelayer 2404 and over exposed surfaces of masking portions 2406 a-c.Oxidation of polysilicon of conductive layer 2404 consumes some of thepolysilicon to form Silicon Dioxide layer 2510. Thus, as Silicon Dioxidelayer 2510 is formed, floating gate layer 2404 is partially consumed anddimensions are reduced. In particular, dimensions of verticalprojections 2408 a-c along the X-direction are reduced. Similarly,dimensions of masking portions 2406 a-c are reduced by oxidation Thethickness of Silicon Dioxide layer 2510 and the amount of polysiliconand Silicon Nitride consumed may be controlled by controlling the totaloxidation time and controlling process conditions. Suitable processesfor oxidizing polysilicon and Silicon Nitride include those that useOxygen radicals to perform oxidation at relatively low temperatures(less than 500 degrees Centigrade). For example, Decoupled PlasmaNitridation (DPN) or Slot Plane Antenna (SPA) plasma processing systemsmay be used to oxidize both polysilicon and Silicon Nitride. In analternative embodiment, resist slimming or other means may be used toform narrow masking portions and vertical projections. Then, adielectric layer may be deposited over the narrowed masking portions andvertical projections to form structures similar to those of FIG. 25.

FIG. 26 shows the structure of FIG. 25 after anisotropic etching to formtrenches 2612 a-b for STI structures. Anisotropic etching leaves SiliconDioxide along sidewalls of vertical projections 2408 a-c and maskingportions 2406 a-c so that sidewall spacers 2510 a-f are formed. SiliconDioxide is etched through between sidewall spacers 2510 a-f and aportion of the underlying conductive layer 2404 is then etched throughto form separate conductive portions 2404 a-c. Subsequently, substrate2400 is etched to form trenches 2612 a-b with sidewall spacers 2510 a-fdefining locations of the sides of trenches 2612 a-b. Etching of SiliconDioxide layer 2510 and underlying substrate 2400 may be performed asseparate etch steps using different chemistry. Trenches 2612 a-b arethen filled with Silicon Dioxide. Silicon Dioxide may fill trenches 2612a-b beyond the surface of substrate 2400 so that Silicon Dioxide fillsthe slot between sidewall spacers 2510 a-f (also formed of SiliconDioxide). Subsequently, planarization may be performed to remove maskingportions 2406 a-c and Silicon Dioxide down to a certain level. Then anetch is performed to remove additional Silicon Dioxide.

FIG. 27 shows the result of planarization and additional etching, sothat Silicon Dioxide is removed down to the level of the lower parts ofconductive portions 2404 a-c, leaving STI structures 2716 a-b. In someexamples, Silicon Dioxide may be removed to a different level. Nomasking portions remain at this stage. Planarization may be achievedusing a process that stops only after all material of masking portions2406 a-c is removed. In other examples, some material of maskingportions 2406 a-c may be left after planarization.

Subsequently, a dielectric layer 2818 is deposited over conductiveportions 2404 a-c and a control gate layer 2820 is deposited overdielectric layer 2818 as shown in FIG. 28. Dielectric layer 2818 andcontrol gate layer 2820 are then patterned, as before, to form separateword lines and separate floating gates in a self-aligned manner. STIstructures 2716 a-b are narrower than floating gates formed fromconductive portions 2404 a-c in this example, though a range ofdimensions is achievable by using different oxidation times andconditions to determine the amount of oxide formed and the amount ofunderlying polysilicon consumed. Thus, a memory array like that shown inFIG. 2A (though with narrower STI structures) is formed with individualfloating gates having an inverted-T shape that is similar to that shownin FIG. 2B.

FIG. 29 shows a flowchart that summarizes the process described in FIGS.24-28. First, a gate dielectric layer and a floating gate (FG) layer areformed over a surface of a substrate 2922. Then, a masking layer isformed over the floating gate layer and is patterned to form maskingportions that extend in the Y-direction 2924. The floating gate layer isetched in a pattern established by the masking portions 2926 so thatsome floating gate material is removed, but the floating gate layer isnot etched through at this stage. Then, an oxidation process isperformed 2928 to grow Silicon Dioxide on exposed surfaces of thefloating gate layer and masking portions. Anisotropic etching is thenperformed to form sidewall spacers from the Silicon Dioxide grown 2930.These sidewall spacers provide masking for etching the floating gatelayer into separate floating gate portions and etch the substrate toform STI trenches 2932. Silicon Dioxide is deposited to fill trenches2934 and to fill slots between sidewall spacers. Then, a planarizationstep removes masking portions and some of the Silicon dioxide 2936. MoreSilicon Dioxide is removed by etching, so that upper portions offloating gates are exposed. Silicon Dioxide remains in trenches to formSTI structures. Subsequently, a dielectric layer is deposited and acontrol gate layer is deposited over the dielectric layer 2938. Apatterning process then forms separate word lines and separate floatinggates in a self-aligned manner 2940.

One embodiment disclosed herein is a method of forming an array ofnon-volatile memory cells on a semiconductor substrate surface. Themethod includes forming a plurality of shallow trench isolationstructures that extend in a first direction, forming a plurality offirst conductive portions that extend in the first direction, forming aplurality of second conductive portions, and forming a plurality offloating gates by etching the first conductive portions and the secondconductive portions. Forming a plurality of first conductive portionsincludes: forming a layer of conductive material over the plurality ofshallow trench isolation structures, forming a first slot over anindividual one of the plurality of shallow trench isolation structures,and removing a portion of the layer of conductive material below each ofthe plurality of slots. Forming an individual second conductive portionsincludes: forming a second slot over an individual one of the firstconductive portions, the second slot being narrower than the individualfirst conductive portion in a second direction that is perpendicular tothe first direction, and forming the individual second conductiveportion in the second slot over the individual first conductive portion,the individual second conductive portion in electrical contact with theindividual first conductive portion.

One embodiment disclosed herein is a method of forming an array ofnon-volatile memory cells comprising: forming a plurality of shallowtrench isolation structures that extend in a first direction in asubstrate, forming a gate dielectric layer over the substrate, forming alayer of first conductive material over the gate dielectric layer;forming a first plurality of strips of masking material over the firstconductive material, the first plurality of strips of masking materialextend in the first direction, the first plurality of strips of maskingmaterial have first sidewalls; forming first sidewall spacers on thesidewalls of the first plurality of strips of masking material, firstportions of the first conductive material are exposed after forming thefirst sidewall spacers; removing portions of the layer of firstconductive material that are exposed to form a first plurality ofconductive strips that extend in the first direction, forming a secondplurality of strips of masking material that extend in the firstdirection, the second plurality of strips of masking material havesecond sidewalls; forming second sidewall spacers on the secondsidewalls, a portion of each of the first plurality of conductive stripsis exposed after forming the second sidewall spacers; forming a secondplurality of conductive strips, each individual conductive strip of thesecond plurality of conductive strips is formed over the exposed portionof one of the first plurality of conductive strips, each conductivestrip of the second plurality of conductive strips is in electricalcontact with one of the first plurality of conductive strips; andforming a plurality of floating gates by etching the first plurality ofconductive strips and the second plurality of conductive strips.

In one embodiment nonvolatile memory array stores charge in floatinggates that have an inverted-T shape in cross section along the word linedirection. This shape reduces coupling between adjacent floating gatesin the bit line direction because of the reduced area of opposingfloating gate facets in the bit line direction. The reduction in thedimension of the upper portion of such a floating gate, compared to afloating gate with a rectangular shape, provides more space for acontrol gate and dielectric layer between adjacent floating gates in theword line direction. A memory array with floating gates having aninverted-T shape may be produced using various processes.

One process for forming an inverted-T shaped floating gate forms STIstructures and channel regions that extend in the bit line directionusing masking portions. By forming masking portions using resistslimming, channel regions are made narrower than STI structures.Channels may also be narrower than the minimum feature size (F) of thelithographic process used. Subsequently, a first floating gate layer isformed and additional masking portions with sidewall spacers are used topattern the first floating gate layer into first floating gate portionsthat are wider than underlying channel regions (and may be wider thanF), thus providing a high tolerance for misalignment between the firstfloating gate portions and channel regions. Subsequently, yet anotherset of masking portions and sidewall spacers is formed so that slotsbetween sidewall spacers extend from first floating gate portions.Second floating gate portions are formed in the slots. Subsequently, adielectric layer and control gate layer are formed over the floatinggates and an etch is performed to separate the control gate layer intoword lines and, at the same time, separate floating gate portions intoindividual floating gates.

Another process for forming an inverted-T shaped floating gate forms afirst floating gate layer and then uses masking portions over the firstfloating gate layer to establish locations for STI structures so thatSTI structures are self aligned to first floating gate portions formedfrom the first floating gate layer. STI structures have sidewalls thatextend vertically to a level higher than first floating gate portions.Sidewall spacers are formed on these sidewalls so that sidewall spacersleave slots over first floating gate portions. Second floating gateportions are formed in these slots so that they are self aligned to thefirst floating gate portions. Subsequently, sidewall spacers are removedand STI structures are partially etched back. A dielectric layer and acontrol gate layer are deposited over the floating gate portions. Thedielectric layer, control gate layer and floating gate portions are thenetched together so that word lines are formed that are self aligned tofloating gates.

Another process for forming an inverted-T shaped floating gate forms afloating gate layer with masking portions extending in the bit linedirection. The floating gate layer is partially etched using the maskingportions to cover parts of the floating gate layer that then formvertical projections when unmasked portions of the floating gate layerare removed. The floating gate layer is not etched through by thispartial etching. Subsequently, sidewall spacers are formed on thesidewalls of vertical projections by an oxidation process that reducesthe thickness of vertical projections. Then, these sidewalls are used asa mask to etch through the floating gate layer, thus forming separatefloating gate portions. Sidewall spacers are also used as a mask foretching into the underlying substrate to form STI trenches. SiliconDioxide is added to fill the trenches. Masking portions and sidewallspacers are removed and a dielectric layer and a control gate layer aredeposited. The control gate layer, dielectric layer and floating gateportions are then etched together to form word lines that are selfaligned to floating gates.

Although the various aspects of the present invention have beendescribed with respect to exemplary embodiments thereof, it will beunderstood that the present invention is entitled to protection withinthe full scope of the appended claims.

1. A method of forming an array of non-volatile memory cells on asemiconductor substrate surface, comprising: forming a plurality ofshallow trench isolation structures that extend in a first direction;forming a plurality of first conductive portions that extend in thefirst direction, the forming a plurality of first conductive portionsincludes: forming a layer of conductive material over the plurality ofshallow trench isolation structures; forming a first slot over anindividual one of the plurality of shallow trench isolation structures,a plurality of first slots are formed; and removing a portion of thelayer of conductive material below each of the plurality of slots;forming a plurality of second conductive portions, forming an individualone of the second conductive portions includes: forming a second slotover an individual one of the first conductive portions, the second slotbeing narrower than the individual first conductive portion in a seconddirection that is perpendicular to the first direction; and forming theindividual second conductive portion in the second slot over theindividual first conductive portion, the individual second conductiveportion in electrical contact with the individual first conductiveportion; and forming a plurality of floating gates by etching the firstconductive portions and the second conductive portions.
 2. The method ofclaim 1 wherein the forming a first slot over an individual one of theplurality of shallow trench isolation structures includes: formingmasking portions over the layer of conductive material, the maskingportions having sidewalls that extend in the first direction; andforming spacers that extend in the first direction along the sidewallsof ones of the masking portions, the plurality of spacers overlying thelayer of conductive material.
 3. The method of claim 1 wherein theforming a second slot over an individual one of the first conductiveportions includes: depositing a masking layer over the first conductiveportions; patterning and etching the masking layer to form a pluralityof mask portions having sidewalls that extend in the first direction;and forming sidewall spacers on the sidewalls of the plurality of maskportions.
 4. The method of claim 1 further comprising forming adielectric layer and a control gate layer over the first conductiveportions and the second conductive portions.
 5. The method of claim 4wherein forming the plurality of floating gates by etching the firstconductive portions and the second conductive portions includes using amask pattern, further comprising patterning the dielectric layer and thecontrol gate layer using the mask pattern.
 6. A method of forming anarray of non-volatile memory cells, comprising: forming a plurality ofshallow trench isolation structures that extend in a first direction ina substrate; forming a gate dielectric layer over the substrate; forminga layer of first conductive material over the gate dielectric layer;forming a first plurality of strips of masking material over the firstconductive material, the first plurality of strips of masking materialextend in the first direction, the first plurality of strips of maskingmaterial have first sidewalls; forming first sidewall spacers on thesidewalls of the first plurality of strips of masking material, firstportions of the first conductive material are exposed after forming thefirst sidewall spacers; removing portions of the layer of firstconductive material that are exposed to form a first plurality ofconductive strips that extend in the first direction; forming a secondplurality of strips of masking material that extend in the firstdirection, the second plurality of strips of masking material havesecond sidewalls; forming second sidewall spacers on the secondsidewalls, a portion of each of the first plurality of conductive stripsis exposed after forming the second sidewall spacers; forming a secondplurality of conductive strips, each individual conductive strip of thesecond plurality of conductive strips is formed over the exposed portionof one of the first plurality of conductive strips, each conductivestrip of the second plurality of conductive strips is in electricalcontact with one of the first plurality of conductive strips; andforming a plurality of floating gates by etching the first plurality ofconductive strips and the second plurality of conductive strips.
 7. Themethod of claim 6 further comprising forming a dielectric layer and acontrol gate layer over the first plurality of conductive strips and thesecond plurality of conductive strips.
 8. The method of claim 7 furthercomprising patterning the dielectric layer and the control gate layerusing a mask pattern that is used when etching the first plurality ofconductive strips and the second plurality of conductive strips.